A synchronous machine requires no exciting current because a magnetic flux is established by a magnet incorporated in a rotor, as compared to an induction machine commonly widely used in various fields, and generates no secondary copper loss because no electric current flows to the rotor as in the induction machine. Therefore, application of the synchronous machine to electric vehicles has been examined to improve their efficiency.
An electric vehicle runs while connecting a plurality of cars each having a power converter and a synchronous machine incorporated therein to form a unit. Therefore, even if a short-circuit fault occurs in switching elements that constitute a power converting apparatus of a part of the cars while these cars are running, the electric vehicles can run continuously by other normal power converters and synchronous machines. Accordingly, a short-circuit current due to an induced voltage of the synchronous machine continuously flows to a fault location (a short-circuited portion) of a faulty power converter. Consequently, if this state is left unsolved, a large short-circuit current flows, and thus fault locations of the power converter may increase or the synchronous machine may generate heat or burn out because the short-circuit current also flows to the synchronous machine.
As a measure against such a fault, Patent Literature 1 mentioned below discloses a power converting apparatus including: a power converter that has a plurality of on/off-controlled switching elements and converts a DC voltage to an AC voltage of an arbitrary frequency to drive an AC motor; a current detector that detects an output current of an opening/closing unit for AC blocking, which is connected between the power converter and a synchronous machine, and an output current of the power converter; and a controller that performs on/off control of the switching elements in the power converter and opening/closing control of the opening/closing unit based on at least the current detected by the current detector, for example. The controller includes a fault determining unit that determines whether any of the switching elements has a short-circuit fault or remains in an ON operation state based on a current value detected by the current detector and outputs a signal indicating a determination result, and a gate signal generator that outputs a control signal to the power converter according to the signal from the fault determining unit. When the signal from the fault determining unit indicates that any of switching elements belonging to an upper arm connected to a positive terminal has a short-circuit fault or remains in an ON operation state and that any of switching elements belonging to a lower arm connected to a negative terminal has a short-circuit fault or remains in the ON operation state, the gate signal generator outputs an ON control signal for setting all the switching elements to the ON operation state to the power converter. Accordingly, a fault current flowing between the power converter and the motor can be blocked when the fault current includes a DC component in which continuous current zero does not appear, irrespective of a type of a fault occurring in the power converting apparatus, thereby enabling to prevent the increase of fault locations in the power converter.
Patent Literature 2 mentioned below describes a measure to return a DC component as an average value of currents of a synchronous machine to zero by setting, when a short-circuit fault occurs in a switching element, the switching element to an ON operation state so that a motor terminal of a high-potential P-side arm or a low-potential N-side arm is short-circuited to include a phase having a short-circuit fault, thereby obtaining a three-phase short-circuit state.